The invention relates to the construction of a cellulosic chips material treating vessel, and for a method of treating chips material or the like utilizing such a vessel, that overcomes practical problems heretofore encountered with certain prior art vessels.
Conventionally, upright cylindrical vessels for treating chips material or the like have a scraper disposed at the vessel bottom, and rotatable about a vertical axis, for effecting a uniform descending motion of the chips material over the entire cross-section of the vessel, and assuring proper movement of the chips material to the vessel outlet. Such scrapers are conventionally used in pulp digesters, impregnation vessels, and the like. It has been found according to the present invention that under some particular circumstances undesirable losses of pulp strength occur in treatment vessels. According to the present invention, it has been determined that the source of the pulp strength loss under some circumstances is as a result of the mechanical action exerted by the rotating scraper on the chips in the presence of hot alkali liquid substantially at, or near, cooking temperatures. Such a problem is most prevalent at the bottom of the first treatment vessel of a 2-vessel hydraulic digester system, such as disclosed in U.S. Pat. No. 4,104,113, the disclosure of which is hereby incorporated by reference herein. At the first vessel bottom portion in such a system hot liquid is introduced under pressure to flush (sluice) the chips material out of the vessel bottom to the top of the second treatment vessel.
According to the present invention the loss-of-pulp-strength problem encountered in conventional systems can be eliminated while still allowing the scraper to perform its desired functions of effecting uniform descent of the chips material and proper movement thereof to the vessel outlet. This is accomplished according to the present invention by isolating the scraper from the heated liquid flowing into the vessel bottom portion so that the temperature at the area of the scraper is at a first temperature which is low enough so that substantial loss of strength of the fiber material does not result from the mechanical action exerted thereon by the scraper. Such isolation is accomplished by providing a false bottom above the vessel bottom portion, with the scraper mounted for rotation just above the false bottom and a central opening being provided in the false bottom to allow chips to descend from the vessel to the vessel bottom portion containing the outlet. Treatment liquid is introduced into the vessel bottom portion below the false bottom to entrain the chips materials or the like therein and flush the entrained chips out the outlet. The introduced treatment liquid is at a second temperature substantially greater than the first temperature, which second temperature is great enough so that if mechanical action from the scraper were exerted upon chips entrained in liquid at the second temperature substantial loss of strength of the pulp would ensue. The first temperature is preferably maintained at about 230.degree. to 300.degree. F., and the second temperature is about 335.degree. to 350.degree. F. (approximately at cooking temperature for effecting digestion of the cellulosic fiber material).
It is the primary object of the present invention to provide an improved method and apparatus for the treatment of cellulosic fiber material (especially in a 2-vessel hydraulic digester system) whereby substantial loss of pulp strength as a result of mechanical action exerted on the material in the presence of hot alkali cooking liquor does not occur. This and other objects of the invention will become clear from an inpsection of the detailed description of the invention, and from the appended claims.